A hollow blow molded thermoplastic, such as a thermoplastic polyester or a biaxially oriented polyethylene terephthalate resin, "PET", container, commonly used to contain food or beverage, has excellent physical properties, durability, and a wide range of applications. However, when used in hot-fill applications, certain portions of the container which are not subjected to the biaxial orientation during the blow molding process, such as the neck area seal edge or thread, commonly referred to as the finish area, are found to soften and deform in an undesirable manner because the temperature of the food or beverage fill is often above the glass transition temperature of the plastic. Many methodologies and processes have been tried in attempts to strengthen the neck area of such containers by enhancing the density of the resin and opacifying and whitening the neck due to the crystallization of the thermoplastic resin by heat treating the neck.
Strengthening the neck area of a thermoplastic container greatly increases the craze resistance of the neck area, improves the mechanical rigidity and increases impact resistance, wear resistance, and external pressure resistance of the container. However, a common side effect of such a thermal treatment of the neck area of the container is an undesirable deformation of the neck area, thus leading to problems with capping and sealing the container.
U.S. Pat. No. 4,476,084 provides a solution to the problem of deformation during heat treating by placing a cold die pin or plug into the bore of the neck prior to heating. The cold die pin assists in maintaining the proper size and shape of the neck during the crystallization process. Others have found similar solutions. See, for example, U.S. Pat. Nos. 4,388,356, 4,379,099, 4,572,811, 4,590,021 and 5,261,545. The use of such cold plugs and dies however have led to problems when speed of manufacture is a priority. Inefficient heating, failure to properly position the plug and deformation of the thermoplastic container due to the weight of the plug have commonly been experienced, thereby leading to a further search for a fast, efficient way of crystallizing the neck area of the thermoplastic container.